1. Field of the Invention
The present invention relates to composite substrates.
2. Description of the Related Art
Surface acoustic wave devices (SAW devices) are widely used as bandpass filters in communication equipment such as cellular phones. SAW devices are manufactured, for example, from a composite substrate including a piezoelectric substrate, such as a lithium tantalate (LT) or lithium niobate (LN) substrate, and a support substrate, such as a sapphire or silicon substrate, that are joined together. A piezoelectric substrate, such as an LT or LN substrate, has a large electromechanical coupling coefficient and is therefore advantageous for achieving filter characteristics over a wide band, although this substrate is disadvantageous in that it has poor temperature stability. On the other hand, a support substrate, such as a sapphire or silicon substrate, has superior temperature stability, although this substrate is disadvantageous in that it has a small electromechanical coupling coefficient. In contrast, a composite substrate manufactured by joining the two substrates together is advantageous in that it has a large electromechanical coupling coefficient and superior temperature stability.
PTL 1 discloses a composite substrate manufactured by activating joining surfaces of a piezoelectric substrate and a support substrate with an ion beam, neutralized beam, or plasma of an inert gas or oxygen and then joining the two substrates together at room temperature or under heating at 100° C. or lower. PTL 1 also discloses that the joining is followed by annealing at a relatively low temperature, i.e., 200° C. or lower, to further improve the adhesion strength between the two substrates. PTL 1 also discloses that a composite substrate formed by activating the joining surfaces of the two substrates with an irradiation beam of argon atoms and then joining together the joining surfaces of the two substrates includes an amorphous layer between the substrates. Such a composite substrate including an amorphous layer between the substrates is also disclosed in PTL 2.